Quantum cryptography, or quantum key distribution (QKD) as it is more accurately known, exploits fundamental quantum properties of single photons of light in order to guarantee the security of information transmitted over optical communication networks. The technique was disclosed by IBM in the mid-1980s and has since been the subject of much research activity which has recently culminated in the first commercial QKD product releases (see, for example, www.idquantique.com and www.magiqtech.com). To date, most research and development in the QKD area has focused on applications over intermediate reach (≧100 km), point-to-point fibre links with the general goals of increasing both the distance and bit rates supported by the QKD system. In contrast, the possibility of exploiting QKD to protect smaller-scale (˜10 km), fibre-to-the-home/business (FTTx) access networks has received comparatively less attention since the original concept demonstration.
FTTx has been envisaged for a long time as an attractive future access technology for delivering high bandwidths to customers. However, until recently the development and widespread deployment of copper-based broadband solutions such as digital subscriber line and cable modem had slowed down its introduction. Now, demand for new high bandwidth services such as interne protocol (IP) television and video on demand, as well as changing competitive and regulatory forces, are beginning to drive the deployment of fibre access networks around the world, “Driving Fibre Closer to the home”, K. Twist, Nature Photonics, Vol. 1, 149-150. Japan is the current world leader with more than 10 million FTTx customers, but significant deployments are also underway in the USA, Korea and, more recently, in Europe. One of the most attractive optical access network architectures is the passive optical network (PON), which is highly cost-effective because the network infrastructure is c hared by many customers and has no active components, such as electronic switches or routers, in the path between the telecommunication provider's central office or local exchange and the customer. The first generations of PONs are now standardised and commercially available, the most advanced PONs typically offer 2.5 Gbit/s or 1 Gbit/s on the downstream channel (1490 or 1550 nm wavelength) and ˜1 Gbit/s on the upstream channel (1310 nm wavelength). This available bandwidth is shared via passive optical splitters and a time-division multiple access (TDMA) protocol, over a reach of around 10 km.
In order to be cost-effective, QKD channels must typically operate over the same fibre infrastructure as conventional optical communication channels. These conventional channels may carry ordinary unencrypted data, data that is encrypted using the keys exchanged on the QKD channel, timing and control information that is required for operation of the QKD channel, or a combination of the above. U.S. Pat. No. 5,675,648, assigned to British Telecom, describes this combined conventional/quantum channel transmission.
U.S. Pat. No. 5,768,378, assigned to British Telecom, describes a QKD implementation on an example (multi-user PON) fibre network infrastructure. However, QKD employs (at most) a single photon per bit of transmitted information; a value that is approximately 7-8 orders of magnitude lower than for a typical conventional optical communication system. The main problem with this implementation is that cross-talk from conventional data channels operating over the network can easily prevent effective operation of the QKD channel unless suitable complex cross-talk mitigation schemes are employed. This is particularly relevant in multi-channel, wavelength-division-multiplexed (WDM) systems where the cross-talk is dominated by Raman scattering present. The latter converts a proportion of the photons from each conventional data channel to new wavelengths, spread over a wide (˜300 nm) range centred on the original channel wavelength. If these Raman photons lie within the wavelength band allocated to the QKD channel then cross-talk will occur. The level of cross-talk, its impact on the performance of the QKD system and the problems associated with Raman scattering is described in detail in the paper “Backscattering limitation for fiber-optic quantum key distribution systems” by Subacius, D.; Zavriyev, A.; Trifonov, A., Applied Physics Letters 86, 011103 (2005).
Two main approaches to system design have been employed previously to reduce Raman cross-talk is as follows:
QKD Channel Out of Raman Band
The QKD channel wavelength λQKD is chosen to lie outside of the Raman bands of the conventional channels. A relatively low-cost, broad-band, conventional optical filter can then be used to block the Raman photons from entering the QKD receiver. Such an approach is described in the paper “Experimental characterization of the separation between wavelength-multiplexed quantum and classical communication channels”, Nweke, N. I.; Toliver, P.; Runser, R. J.; McNown, S. R.; Khurgin, J. B.; Chapuran, T. E.; Goodman, M. S.; Hughes, R. J.; Peterson, C. G.; McCabe, K.; Nordholt, J. E.; Tyagi, K.; Hiskett, P.; Dallmann, N., Applied Physics letters 87, 174103 (2005) published on Oct. 21, 2005. This scheme is illustrated in FIG. 1, which shows a schematic diagram of the optical spectrum at the output of an optical fibre carrying a high intensity, conventional data channel (1) and a low intensity, quantum key distribution channel (QKD) (3). Spontaneous Raman scattering in the fibre converts a proportion of the photons from the conventional channel to new frequencies resulting in a broad spectral ‘pedestal’ (2). A filter with an appropriate broad passband (4) can then be used to isolate the QKD channel from the conventional channel and its Raman background.
QKD Channel In Raman Band
In many optical communication systems, wavelength division multiplexing (WDM) techniques are employed to allow a single fibre to support multiple conventional data channels. This situation is illustrated schematically in FIG. 2 where an additional data channel (5) and associated Raman scattering (6) are shown in the output spectrum. In this case there is no Raman-free region of the spectrum and the QKD channel must operate in the presence of a significant level of Raman-induced cross-talk from the conventional channel (5). Without mitigation this cross-talk would generate significant errors in the QKD channel thus preventing secure operation. In practice, the conventional data channels may be closely spaced (a typical frequency separation is 100 GHz) and large in number (64 or more) so that the Raman spectra of multiple channels overlap at λQKD. Nevertheless, it may still be possible operate the QKD channel if a specifically-tailored, narrow band-pass optical notch filter centred on λQKD with very high out-of-band blocking is used to suppress a large fraction of the Raman scattering. Such an arrangement is described in U.S. Pat. No. 7,248,695, Beal et al. This is effective because of the broad bandwidth of the Raman scattering when compared with a typical QKD channel bandwidth (<1 nm), but nevertheless some Raman photons will still reach the QKD detector. In core or metro networks, the cost of the optical communication system is effectively shared across many customers, but in an access PON there is no cost-sharing of the customer-end optical system and it is critical, therefore, that component costs are as low as possible. The scheme of U.S. Pat. No. 7,248,695 uses a non-standard narrow-band filtering scheme in which either filters or quantum channel source or both would require accurate temperature control and wavelength locking in order that the source and filter wavelengths remain matched as the temperature varies. This would be impractical to implement in an access PON where, depending on the location of the quantum channel receiver, each customer would require either an expensive temperature controlled, high specification, non-standard notch filter or an expensive temperature controlled photon source.
Other systems that describe QKD implementations are described in EP 1 633 076, Nippon Electric Co., and US 2006/198521, Young et al. EP 1 633 076 discloses a system where a quantum channel and a classical channel are multiplexed with multiplexer and demultiplexer on a single optical transmission line and information is transmitted from a transmitter to a receiver through the quantum channel, the classical channel is inhibited from affecting the quantum channel. EP 1 633 076 primarily teaches an optimised filtering scheme and power control to reduce the mean power of the classical conventional channel when the quantum channel is on.
US 2006/198521 discloses a method of synchronizing the operation of a two-way QKD system by sending a sync signal (SC) in only one direction, namely from one QKD station to another QKD station. The one-way transmission greatly reduces the amount of light scattering as compared to two-way sync signal transmission. The method includes phase-locking the sync signal at the first QKD station and dithering the timing of the quantum signals so as to operate the QKD system in three different operating states. The number of detected quantum signals is counted for each state for a given number of detector gating signals. The QKD system is then operated in the state associated with the greatest number of detected quantum signals. This method is rapidly repeated during the operation of the QKD system to compensate for timing errors to maintain the system at or near its optimum operating state. The method allows for only having to adjust the timing of a single timed element-namely, the quantum laser-to compensate for timing variations, rather than having to adjust the timing of all or some of the timed elements in the QKD system. The method disclosed is not directed to Raman suppression.
However, these approaches suffer from two main problems. First, in some network applications the out of band approach cannot be employed because no Raman-free wavelength windows exist for conventional channels operating in the 1250 to 1700 nm spectral region. Secondly, for the in band approach the notch filter is expensive to implement as narrow band-pass filters with high out of band blocking are costly to manufacture and both the filters and QKD optical source will require temperature control in order that the wavelengths remain precisely matched.
There is therefore a need to provide a quantum cryptography system and method to overcome the above mentioned problems.